Ductless fume hood with improved filter monitoring system and extended filter life

ABSTRACT

An improved ductless fume hood with an improved system for monitoring filter life and an improved design for extending filter life is disclosed. The improved system for monitoring filter life consists of sensors upstream and downstream of the hood filter. A comparison of the readout of the upstream and downstream sensors allows for calculation of the efficiency of the filter. An alarm may then be triggered when the filter efficiency falls below a pre-determined value. The improved design of the ductless hood is the inclusion of a novel diffuser located upstream of the hood filter that allows for even filter loading. Even filter loading allows more of the volume of the filter to be used, extending filter life.

FIELD OF THE INVENTION

The present invention relates to an improved ductless fume hood with animproved system for monitoring filter life and an improved design forextending filter life.

BACKGROUND OF THE INVENTION

Ductless, or filtration, fume hoods are a specific type of fume hoodthat use a filtration system to remove contaminants from an air stream.Ductless hoods operate by simply forcing contaminated air from the hoodenclosure through a filter to remove chemical vapors before returningthe air to the laboratory environment.

Ductless hoods have several convenient advantages over laboratoryinstalled, ducted hoods. They are mobile and portable and have minimalinstallation costs as they do not have to be connected to a duct system.They are environmentally friendly, as no toxic gases are released intothe atmosphere. Ductless hoods also have very low operating costs, as noconditioned air is removed from the laboratory.

Because of the advantages listed above, ductless hoods are popular withacademic laboratories, especially those with limited budgets. Asductless hoods are able to be operated anywhere in the laboratory andoften are made transparent on all sides, they are ideal for teachingdemonstrations, allowing students to surround the hood. Ductless hoodshave also grown in popularity in industrial laboratories, where they canbe used for specific projects with low costs.

The main drawback of ductless hoods is the potential release of toxicgases into the laboratory because of filter saturation and breakthrough.While most of the advantages of using ductless hoods are derived fromthe re-circulating of air from the hood back into the laboratory, thisre-circulation means that the air exiting the hood must be filtered atall times. Most ductless hoods use an activated carbon filter as theirfiltration system. Although activated carbon is highly adsorbent, theactivated carbon particles eventually become saturated. When theactivated carbon becomes saturated through the thickness of the filter,chemical vapors are no longer adsorbed and begin to break through intothe exhaust.

The primary inconvenience of operating a ductless hood is the need tomonitor the hood filter to ensure the safety of those working in thelaboratory. If the exhaust concentration of a specific compound exceedsthe allowed limits set by the United States Occupational Health andSafety Administration (OSHA) or other local limits, then the filter mustbe changed. Prior designs have conventionally employed a timer thatsounds an alarm every 60 hours of operation time to notify the user thatit is time to check the condition of the filter. However, studies haveshown that these arbitrary alarms rarely coincide with the actual timingof filter saturation. This is not surprising, as this arbitrary methodof filter monitoring does not take into account the actual use of thehood while it is running.

Because of the unreliability of the 60 hour alarms, they are oftenignored, leaving the user to test the condition of the filter wheneverthe user feels it might be necessary. In most cases, the user will waituntil a detectable odor develops in the laboratory, which is often thepoint at which the concentration of the compound in the air has alreadyexceeded the OSHA limits. In other cases where the compound being usedis odorless, the user is forced to be very vigilant in checking filterefficiency, and often spends a great deal of time performing tests thatare not necessary. Better methods of filter monitoring are needed tomaintain the safety of the hood operator and others in the laboratorywithout imposing inconvenient requirements that the filter be checkedmore often than necessary.

Other ductless hood designs, such as that described in U.S. Pat. No.4,946,480, which is hereby incorporated by reference herein, haveattempted to solve this problem by installing a gas sensor downstream ofthe filter to detect the concentration of compounds in the filterexhaust. This effort has largely proven futile as it is not possible tomonitor the hundreds of different compounds used in a laboratory withjust one sensor. The sensors used in ductless hoods are typically broadrange detectors without any specificity for particular compounds.Although control system read-out can be obtained for the exhaust gasconcentration, it is difficult to correlate this read-out to an actualconcentration of an actual gas, and report to the user if thatconcentration actually exceeds the OSHA exposure limits. Much of thisphenomenon comes from the fact that the sensor has widely variedsensitivity to different gases. This varied sensitivity makes it verydifficult to choose a level of detection for the sensor at which thealarm should be triggered, especially in a situation when multiple typesof chemicals are to be used in the hood. An improved ductless hoodfilter monitoring system would greatly improve on the safety and ease ofuse of ductless hoods.

With both of the above filter monitoring methods, it is still necessaryto perform air sampling tests to confirm that the filter is actuallycompromised. These tests usually involve use of a gas detection tubecontaining a color change reagent specific to the gas to be detected.Whenever a filter alarm sounds, the user must stop work and take thetime to sample the exhaust air using a hand pump before deciding if afilter change is actually necessary. More convenient methods fordetermining filter life are necessary to simplify compliance with safetyregulations.

Another current problem with ductless hood systems is uneven filter use.The filter of a ductless hood is usually about the same size as theceiling of the enclosed hood area to allow for better filtration of theentire hood. However, most work is done in the center of the hood,meaning that the majority of chemical vapors come in contact with thecenter of the filter. This causes the center of the filter to quicklybecome saturated and allows for the breakthrough of chemical vapors eventhough the areas of the filter near the enclosure walls are largelyunused. A device that would cause even loading of a ductless hood filterwould greatly extend the lifetime of the filter, making ductless hoodseven more convenient to use.

Therefore, there remains a need for a ductless hood that extends thelife of the filter and simplifies compliance with safety regulations.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a ductless fume hoodwith an improved system for monitoring filter life.

Another object of the present invention is to provide a system formeasuring the filter efficiency of a ductless hood.

The system of the present invention provides a gas sensor upstream and agas sensor downstream of the hood filter. The upstream sensor measuresthe concentration of gases entering the filter while the downstreamsensor measures the concentration of gases exiting the filter. Data fromthe sensors is sent to a microprocessor, which calculates the efficiencyof the filter based on the change in the concentration of gases betweenthe inlet and exhaust air stream. When the efficiency of filtrationfalls below a specific, pre-determined percentage (for example 90%), themicroprocessor sounds an alarm to warn the user that the filter iscompromised.

The filter monitoring system of the present invention provides thedistinct benefit of being able to accurately monitor efficiency offiltration regardless of the chemicals being used in the hood. Becausesensors of the same type are used both upstream and downstream of thefilter, they will exhibit the same response (i.e. voltage read-out to amicroprocessor) regardless of the chemical vapors present in the airstream.

A further advantage of the upstream and downstream sensor system is thatthere are certain conditions under which filters that are reaching theirmaximum capacity begin to desorb. In such conditions, the concentrationreadings of the downstream sensor will be higher than the concentrationreadings of the upstream sensor. When such a condition occurs, themicroprocessor can sound an alarm to let the user know that the filteris compromised before the expected end of its life. This is a clearadvantage over the current systems with only a downstream sensor, whichwould not be able to detect such an event.

Another object of the filter monitoring system of the present inventionis to allow a more accurate assessment of the actual exposure of thefilter to chemical vapors by placing a sensor upstream of the filter.Instead of an alarm sounding after every 60 hours of use, the system ofthe present invention will allow for the prediction and warning of theend of filter life by detecting the approximate concentration ofchemical vapors the filter has been exposed to and the rate of change offilter efficiency.

A further object of the present invention is to provide an improved fumehood comprising a novel diffuser that allows for even filter loading.The diffuser contains a plate of metal or other material with a seriesof holes that is positioned upstream of the filter. The diffuser causesthe chemical vapors in the air stream to disperse over the entire airstream, regardless of the location of the source of the vapors in thehood. This effectively increases the volume of the filter that isexposed to the vapors which, in turn, extends filter life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an embodiment of a ductless hood apparatus of theinvention.

FIG. 2A is a schematic front view of the embodiment of the inventionshown in FIG. 1.

FIG. 2B is a schematic cross-sectional view of the ductless hoodapparatus of FIG. 2A taken along line II.

FIG. 3 is an electrical circuit diagram of a preferred embodiment forthe control system of the invention.

FIG. 4 is a schematic view of an embodiment of the main board of theinvention.

FIG. 5 is a flow diagram showing an embodiment of a microprocessorprogram of the invention.

FIG. 6A is a schematic showing the air flow and filter wear patterns ina conventional ductless hood.

FIG. 6B is a schematic showing the air flow and filter wear patterns ina ductless hood of the invention.

FIG. 6C is a schematic of a preferred embodiment of the diffuser of theinvention.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

The ductless fume hood apparatus of the present invention contains animproved system for monitoring filter life and a novel diffuser causingeven filter loading. An aspect of the present invention is that theefficiency of the filter of the hood can be easily and reliablymonitored. A further aspect of the present invention is that thediffuser causes chemical vapors from the hood to be diffused evenlyacross the surface of the filter, causing even filter loading andincreasing filter life. Although containment of chemical vapors isreferred to throughout the application, an embodiment of the presentinvention could also be used for the containment of biological vaporswith appropriate filters and sensors.

Referring to the drawings, throughout this description, like elementsare referred to by like numbers as shown in the drawings.

One embodiment of the apparatus of the present invention is a ductlesshood 1 as shown in FIG. 1. A blower 3 is used to pull air from thelaboratory into the hood enclosure 5, where it will carry chemicalvapors present in the hood up through the filter 7. Filtered air is thenpulled into the blower compartment 9, through the blower 3, and pushedout the top of the hood back into the laboratory environment.

A front view schematic of an embodiment of the present invention shownin FIG. 1 is shown in FIG. 2A. The ductless hood 1 comprises a hoodenclosure 5 bounded on three sides, preferably by tempered glasswindows, 11 and on the front by an adjustable sliding sash window 13,also preferably made of tempered glass. The sliding sash window 13adjusts to various heights by sliding up into the blower compartment 9to allow the hood operator access to the hood enclosure 5. The bottomsurface of the hood enclosure 5 is a work surface 15, preferably made ofstainless steel, that allows for easy maintenance and clean up ofspills. The internal back wall of the hood enclosure 5, preferably has anumber of electrical outlets 17 to allow for the use of laboratoryequipment inside the hood enclosure 5. The ceiling of the hood enclosure5 is formed by the bottom surface of the blower compartment 9. Themicroprocessor control panel 19 located on the blower compartment 9 isused for monitoring the filter sensors and adjusting the speed of theblower fan.

A side cross sectional view of FIG. 2A taken along line II is shown inFIG. 2B. The hood enclosure 5 is as described above for FIG. 2A. Thewindows 11 on the sides of the enclosure contain service fixtureopenings 21 that allow for attaching various laboratory equipment to thehood enclosure 5. The hood enclosure 5 may be lit by fluorescent lights23. The blower 3 creates an air stream up from the hood enclosure 5through the diffuser 24, past the upstream gas sensor 25 and airflowsensor 27. The air stream then passes through the optional pre-filter 29and into the filter 7 where harmful vapors are removed. Finally, the airstream passes the downstream gas sensor 31, through the blower 3 and outthrough the optional backup filter 33. The electronics of the system arecontained in the electronics panel 35, which is preferably located onthe front of the blower compartment 9 and are controlled by themicroprocessor control panel 19.

In one embodiment of the filter monitoring system, the gas sensors arevolatile organic compound sensors, examples of which are described inU.S. Pat. Nos. 6,565,812, 6,499,335 and 6,128,945, which are herebyincorporated by reference herein. It will be obvious to one of skill inthe art that various types of gas sensors may be used within the scopeof the present invention. It is of primary importance that the upstreamand downstream gas sensors are of the same type, because the readout ofthe two sensors will be compared by the microprocessor to determine thefilter efficiency. However, the invention also contemplates manydifferent sensor types, with the difference taken into account inmonitoring filter efficiency.

The filters used in the ductless fume hood of the invention arepreferably activated carbon filters. Activated carbon filters aresuitable for use with many chemicals, but their use is dependant on anumber of factors. In general, chemicals with a relative molecularweight over 30 g/mol and a boiling point higher than 60° C. (140° F.)can be adsorbed with relatively high efficiency by active carbonfiltration. It is also generally true that larger molecules adsorbbetter than smaller molecules and that less soluble compounds adsorbbetter than more soluble compounds.

There are several factors that influence the adsorption of organiccompounds to activated carbon filters. Branch chain organics areadsorbed more easily than straight chain organics, while unsaturatedorganics (those containing double or triple carbon bonds) are adsorbedmore easily than saturated organics. Polarity is another factor, as lesspolar organic compounds are better adsorbed than highly polar compounds.

Environmental factors may also affect the adsorptive efficiency of thehood. The ambient temperature of the laboratory and the gas must be keptto a minimum, as higher temperatures lead to lower efficiency ofadsorption. Relative humidity must also be kept to a minimum as highrelative humidity can cause the activated carbon filter to adsorb watermolecules in the place of chemical vapors.

The construction of the filter itself also affects the adsorptionefficiency of the hood. A filter with a thicker media bed will have alonger contact time during which a larger amount of chemical vapor willbe deposited in the filter bed by physical adsorption. In order tomaximize this contact time, the air volume passing through the hood andthe filter must be kept to a minimum. However, it is necessary that theinflow velocity of the hood (through the front opening) be maintained ata high enough rate to ensure proper containment of vapors in the hood.

In general, standard activated carbon filters work at an efficiency oftypically higher than 95% across a broad spectrum of chemical compounds.In order to increase adsorptive efficiency for specific applications,specialty carbon filters impregnated with other compounds may also beused. This is commonly known as chemisorption. A non-limiting example ofa specialty carbon filter is a filter in which the carbon media isimpregnated with an oxidizing agent to oxidize formaldehyde andglutaraldehyde fumes. This type of filter is useful for specialtyapplications such as hospital pathology and endoscopy, which involvethese compounds.

Because it is possible to use special filters for specific applications,the filter of the ductless fume hood of the present invention is readilyinterchangeable with other types of specialty filters. It should also beapparent that other, non-activated carbon, filters can be used in theductless fume hood without departing from the scope of the invention.

A preferred embodiment of a control system 37 for a ductless hood of theinvention is shown in the circuit diagram of FIG. 3. The main board 39of the control system monitors input from the various switches andsensors and sends internal messages to the relay board 41 and outputmessages to the user for display on the LCD module 43. Non-limitingexamples of output messages may include feedback such as air flow rateand filter capacity remaining. The user can provide input through themembrane keypad 45, which sends messages through the interface board 47to the main board 39.

The main board 39 receives signals from the system to monitor itsperformance. The upstream sensor 25 and downstream sensor 31 send theirread-out through a printed circuit board (PCB) 49 to the main board 39for calculation of filter efficiency. The main board also receives inputon the air speed moving through the filter from the air flow sensor 27and input on the ambient temperature of the system from the temperaturesensor 51. The system also optionally contains a magnetic switch 53,which detects whether the sliding glass window is at a nominal heightposition.

The main board 39 also sends commands to the relay board 41 whichcontrols various components such as the speed of the blower (PSC motor)3, the alarm system and fluorescent lights 23 for lighting the hoodenclosure. The alarm system is integrated on the relay board 41 and willbe triggered if any unsafe condition is detected by the main board.Non-limiting examples of alarm systems include buzzers, bells, sirensand steady or flashing lights.

The control system 37 diagrammed in FIG. 3 is connected to a powersupply through a power inlet 55. The AC power source is then convertedto DC through the switching mode power supply (SMPS) 57.

The four male-female connectors 58 illustrate the connections betweenall of the components of the system shown in FIG. 3. Connections in thediagram are labeled to represent the connector and pin number throughwhich the connection is made. For example, one of the connectionsbetween the SMPS 57 and the temperature sensor 51 and air flow sensor 27is denoted D10F D10M, which is a connection between the female and malesides of connector D at pin 10. It should be apparent that variousconnectors and types of connections can be used without departing fromthe scope of the invention.

A preferred embodiment of the main board 39 of the present invention isshown in FIG. 4. A microprocessor 59 on the main board 39 interacts withthe other components of the control system through the connectors shownin FIG. 4, including: a relay board connector 61, a LCD module connector63, analog and digital interface connectors 65, 67 for receiving datafrom the gas, air flow, and temperature sensors, and a membrane keypadconnector 69 for user entry. The main board of FIG. 4 also contains aSMPS input 71, a hardware reset switch 73, a battery backup power supply75, a JTAG interface connector for testing the circuit 77, a JP5 circuitfor programming 78 and a DB9 port 79.

In a preferred embodiment, the microprocessor of the present inventionis a microprocessor such as those described in U.S. Pat. Nos. 5,805,909,5,877,641, and 6,828,869, which are hereby incorporated by referenceherein. A non-limiting example of the microprocessor of the presentinvention is one of the Micro-Controller MSP430F series from TexasInstruments, Inc. (Dallas, Tex.). It should be apparent thatmicroprocessors of various types could be used for controlling andmonitoring the ductless hood system of the present invention.

An embodiment of a microprocessor program for monitoring filterefficiency is shown in the flow diagram of FIG. 5. The microprocessorreceives input from the upstream and downstream sensors and computes theefficiency of filtration 80 by comparing the concentration of volatileorganic compounds entering and exiting the filter. If the percentefficiency is found to be below a specific pre-determined number (forexample 90%) 82, then the alarm sounds 84. The user is then given thechoice to replace the filter 86. If the user chooses to replace thefilter the system can be reset and use can be restarted. If the userdoes not replace the filter, the user has the option of checking theexhaust of the hood 88 by manual air sampling. If the user chooses totest the exhaust 90 he/she may then determine whether a toxic level ofgas in the exhaust was reached 92. If a toxic level was not reached, theuser may then adjust the alarm set point to prevent future false alarms94.

The microprocessor program diagrammed in FIG. 5 allows the user tocustomize the alarm set point for specific applications. Once a reliableset point is established, the user will not be required to performmanual air sampling and will be able to rely on the alarm system todetermine when the filter is no longer effective. This program allowsfor the added convenience of being able to rely on an alarm withoutcompromising the user's safety.

It should be apparent that the percent efficiency for the alarm setpoint can be set at varying levels without departing from the scope ofthe invention. The percent efficiency for the alarm set point can bechanged as needed for specific applications. By way of non-limitingexample, it might be desirable to change the percent efficiency of thealarm set point when changing the compound or compounds being used inthe hood. A further non-limiting example would be changing the percentefficiency of the alarm set point when a significantly greater orsmaller amount of a compound is to be used in the hood. Mostimportantly, the percent efficiency of the alarm set point should be setat a level that warns the user when the filter is allowing harmfulvapors to be exhausted from the hood at a concentration that is greaterthan OSHA or local standards.

It should also be apparent that other microprocessor programs could beused for monitoring filter efficiency within the scope of the presentinvention. Any program that is able to compare the input readings fromthe upstream and downstream sensors can be used in an embodiment of thefilter monitoring system.

The microprocessor will constantly monitor the ductless hood system andprovide feedback to the user. Non-limiting examples of feedback that themicroprocessor will provide include:

-   a. Filter efficiency. This is calculated by examining the sensor    outputs of the two sensors (one upstream and one downstream) by    monitoring the reduction of concentration of chemical or other    vapors by the filter. As the ductless hood will not be in continuous    operation, there will be periods of time when the sensors will both    read zero. The microprocessor programming will take this into    account and display the “last known average value” of the filter    efficiency.-   b. Theoretical filter capacity utilization. When the hood is new,    this starts at 100% (capacity remaining). As the filter efficiency    deteriorates from the initial 100% value, to the minimum acceptance    level (for example 90%), the filter capacity remaining will be    pro-rated. For the non-limiting example shown in FIG. 5, when the    filter efficiency is 100%, the capacity remaining is 100% as well.    When the filter efficiency drops to 95%, the capacity remaining will    be 50%.-   c. In certain conditions, the carbon filter can start to desorb when    close to maximum capacity, the filter will emit vapors such that the    concentration on the effluent (downstream) side can be higher than    on the influent (upstream) side. This will create a unique situation    during which the efficiency cannot be calculated (due to a logic    error that the outlet concentration is higher than the inlet    concentration). In such a situation, the second sensor will register    a reading while the first sensor will be zero. This will be an    immediate indication to the control system to inform the user to    change the filter.

In another embodiment of the present invention, the upstream sensor isused to approximate filter life. The upstream sensor provides continuousdata to the microprocessor while the ductless hood is being operated.The data provided by the upstream sensor can be used to determine theapproximate concentration of chemical vapors to which the filter hasbeen exposed. This concentration can be integrated over time todetermine when a filter has been exposed to an amount of chemical vaporthat is close to its saturation point. This way, the user will have amethod to predict when the filter needs to be changed, and may evenchange the filter in advance of the alarm sounding. This would beespecially important to prevent the interruption of complicatedlaboratory procedures when the filter is near saturation.

FIG. 6 shows an embodiment of the diffuser system of the presentinvention. FIG. 6A is a schematic of the pattern of filter saturation ina conventional fume hood. As stated above, the center of the filter 7 ismore highly exposed to chemical vapors due to the air flow of the hoodand the location of the source of vapors in the center of the worksurface. FIG. 6B is a schematic showing how the addition of a diffuser24 to the ductless hood causes changes in air flow that lead to uniformexposure of the filter 7 to chemical vapors.

FIG. 6C is a schematic of a preferred embodiment of the diffuser 24 ofthe present invention. The diffuser 24 in FIG. 6C is a perforated sheetwith a specific pattern of holes that causes the air flow from the hoodenclosure to be evenly spread out over the entire filter 7 surface. Thediffuser 24 of the present invention can be made of metal, plastic orany other material that does not react with the chemical vapors thatwill be produced in the hood.

A preferred embodiment of the pattern of holes for the diffuser 24 isshown in View A of FIG. 6C. In this preferred embodiment, the diffuserholes are circular holes 3 mm in diameter with a 5 mm pitch. It shouldbe apparent that there are other patterns of holes that still fallwithin the scope of the diffuser of the present invention, includingvarious shapes and sizes of holes in different arrangements withdifferent spacings between them. It should be also apparent that adiffuser with any pattern of holes that allows for the air stream comingfrom the hood enclosure to be diffused over the entire surface of thefilter falls within the scope of the present invention.

Specific embodiments of the apparatus of the present invention have beenset forth above. It should be apparent to one of skill in the art thatthere are further variations that fall within the scope of the inventionas set forth in the claims below. A non-limiting example of a variationthat falls within the scope of the invention is a change in the size ofthe apparatus, such as a small desktop hood apparatus or an apparatuslarger than a conventional fume hood. It should also be appreciated thatthe apparatus of the current invention can be used for protecting a userfrom chemical vapors in settings outside of a laboratory, such as in amanufacturing or commercial setting.

1. An apparatus for protecting a user from harmful vapors comprising, anenclosure; a means for producing an air stream to evacuate theenclosure; a filter in the air stream downstream of the enclosure; andsensors placed both upstream and downstream of the filter for monitoringthe efficiency of the filter in removing vapors.
 2. The apparatus ofclaim 1, further comprising a microprocessor to receive signals from thesensors for determining the efficiency of the filter.
 3. The apparatusof claim 1, further comprising a diffuser upstream of the filter.
 4. Anapparatus for protecting a user from harmful vapors comprising, anenclosure; a means for producing an air stream to evacuate theenclosure; a filter in the air stream downstream of the enclosure; and adiffuser upstream of the filter.
 5. A method for determining theefficiency of a filter to remove harmful vapors comprising, positioningsensors capable of detecting a vapor upstream and downstream of thefilter; comparing the upstream sensor readout with the downstream sensorreadout; and triggering an alarm when the filter efficiency falls belowa pre-determined value.
 6. A method of determining the exposure of afilter in an air stream to compounds comprising, positioning a sensorupstream of the filter; detecting the concentration of compounds passinginto the filter; summing up the concentration of compounds over time todetermine the exposure of the filter; and triggering an alarm when thetotal exposure of the filter rises above a pre-determined value.
 7. Amethod for determining when a filter is no longer effective at removinga compound in an air stream comprising, positioning sensors capable ofdetecting the compound upstream and downstream of the filter; comparingthe upstream sensor readout with the downstream sensor readout; andtriggering an alarm when the downstream sensor readout becomes greaterthan the upstream sensor readout.
 8. A ductless fume hood comprising, afirst enclosure above a work surface; and a second enclosure, the bottomsurface of which forms the ceiling of the first enclosure and allows airto pass between the first and second enclosures, said second enclosurecomprising, a diffuser, positioned downstream of the first enclosure; afirst sensor capable of detecting harmful vapors and providing areadout, positioned downstream of the diffuser; a filter capable ofremoving the harmful vapors, positioned downstream of the first sensor;a second sensor capable of detecting harmful vapors and providing areadout, positioned downstream of the filter; and, a means for creatingan air stream capable of evacuating the first enclosure, positioneddownstream of the second sensor, wherein efficiency of the filter inremoving vapors is monitored by comparing the readout of the firstsensor with that of the second sensor.